W
Sheet n°300 - June 2008
Patagonian glacier yields clues for improved
understanding of global climate change
© IRD / Robert Gallaire
lthough ice cores
obtained
from
Antarctica have now
provided more than
800 000 years’ worth of
climate records, analysis
of them alone is insufficient for understanding
the history of climatic
the diverse regions of
the world. Boreholes
drilled during the 1990s
on six glaciers in the tropical zone of the Andean
Cordillera gave rise to a
substantial collection of
data on the changes and
developments of the
tropical climate of the
Southern Hemisphere.
However, no investigation of this type had hitherto been performed at
mid latitudes, leaving a
large gap in documentation on climate. An expedition in 2005 by an IRD
team and its partners1
on the San Valentin glacier in the Chilean part of
Patagonia demonstrated
the potential of that site
for exploring climatic variations of the past. The
analyses gave the first
evidence of influences
from Antarctica and the
Pacific on the Southern
climate of the American
continent, thus indicating the complexity of
the climate system in
this ecologically fragile
region. A follow-up to
this test study, a borehole made in 2007 on the
whole thickness of the
glacier, should provide
the element that was still
missing from the glacier
records on the Southern
Hemisphere’s climate.
© LGGE / IRD / Stéphane Houdier
A
Extraction of an ice core near the San Valentin drilling station.
A better understanding of climate
variations at planetary scale is one of
climate scientists’ crucial concerns.
Stable water isotope analysis, the chemistry of ice cores taken from the Arctic
and Antarctic polar ice caps and of air
bubbles trapped in them now allow a
chronology to be drawn up of the climate
changes that took place over the past
800 000 years. However, those data,
collected at extreme latitudes, are not
enough for understanding climatic interactions operating at the scale of the
whole Earth or of the most densely populated regions. Similar investigations
are needed on glaciers located at lower
latitudes. Scientists have therefore since
the 1990s been undertaking borehole
surveys in the Andean glaciers. The Andes
are particularly suited for sampling climate data concerning the whole of the
Southern Hemisphere owing to their high
altitudes and N-S orientation. Boreholes
on six glaciers of the Andean Cordillera
at tropical latitudes have already yielded
information on South America’s past
climate variability (up to 25 000 years).
However, no study of this type had yet
been conducted in Patagonia, at midlatitudes of the Southern Hemisphere.
During a 2005 expedition by an IRD
team and its Chilean partners on the San
Valentin glacier (Patagonia, 47°S, 4032 m),
a 16 m shallow firn core was extracted
in order to evaluate this site’s potential
as a record of our climate. A borehole at
this latitude should provide the element
still missing from ice field documentation
on the Southern Hemisphere’s climate.
Geographically, it is at the interface
between the tropics and the South Pole
and should contain clues as to how tropical and polar atmospheric circulation
influence this region’s climate.
Preliminary ice core analysis revealed
that the isotopic and chemical tracers
are remarkably well preserved owing
to a sufficiently cold ice temperature
(-11°C). Dating combining determination
of radioactive element levels (tritium,
cesium, americium, lead 210) and the
number of seasonal cycles of chemical
species gave an estimated annual snow
accumulation of about 35 cm. With just
16 m of ice the hope was to obtain a climate record for a period of at best a few
years, but dating showed that the record
in fact went back to the early 1960s.
Combination of oxygen isotope ratio
determinations with those of hydrogen
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CONTACT :
FRANÇOISE VIMEUX
Chargée de recherche
IRD
Unité de recherche
Glaciers et ressources
en eau d’altitude Indicateurs climatiques
et environnementaux
(GREAT ICE)
Address : LSCE
CEA Saclay
Orme les merisiers
Bâtiment 701
91191 Gif-sur-Yvette
France
Tel : 01 69 08 57 71
francoise.vimeux@ird.fr
REFERENCE :
VIMEUX, F., M. DE ANGELIS,
P. GINOT, O. MAGAND,
G. CASASSA, B. POUYAUD,
S. FALOURD, and S. JOHNSEN
(2008),
A promising location
in Patagonia for paleoclimate and paleoenvironmental reconstructions revealed by a
shallow firn core from
Monte San Valentín
(Northern Patagonia
Icefield, Chile), Journal
Geophysical Research,
in press
Doi : 10.1029/2007JD
009502
was then used to estimate the precipitations that feed the San Valentin glacier.
The difference between the isotopic ratios
− the deuterium excess − is linked essentially to the temperature of the oceanic source of the precipitation, making it
possible to differentiate the air masses
coming from the pole, formed above a
cold ocean, from those arising over a
more temperate ocean like the Pacific.
Similarly, a high marine salt concentration in ice means that the precipitation
that feeds the glacier arrives with marine air masses, formed over the Pacific.
Conversely, a low sodium concentration
characterizes continental air masses,
which have travelled for a longer time.
Patagonia was hitherto thought to be
subjected mainly to westerly winds off
the Pacific, but this dual ice core analysis yielded the first evidence that this
region also comes under the influence of
meteorological regimes that arise further
south, in the Antarctic (see Figure).
A second drilling expedition conducted
on San Valentin in 2007 gave the team
the opportunity to drill through the entire
122 m thickness of the glacier. The first
investigations on this second ice core
suggest that it contains a climate record
of several thousand years.
By cross-referring the information
contained in this unique core with those
already obtained for the glaciers lying
further North on the Cordillera, it could
therefore be possible to trace the climate changes in all the whole of the
Southern Hemisphere during the past
few thousand years and thus better anticipate its reactions to global
climate variations.
Grégory Fléchet - DIC
Translation - Nicholas Flay
1. These research investigations
were financed by the French Agence
Nationale de la Recherche (ANR),
and conducted in partnership with
the Centro de Estudios Cientificos
Valdivia (CECS), the Laboratoire
des Sciences du Climat et de l’Environnement (LSCE) at Gif-sur-Yvette, the Laboratoire de Glaciologie
et Géophysique de l’Environnement
(LGGE) of Grenoble and the Laboratoire des Mécanismes et Transferts
en Géologie (LMTG) at Toulouse.
KEY WORDS :
Glacier, climate,
Patagonia
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© IRD / Françoise Vimeux
Sheet n°300 - June 2008
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Relation between the «warm» sources of precipitation (d > 10‰) and high marine inlets indicating air masses
coming from the Pacific. In contrast, “cold” sources of precipitation (d < 9‰) and low marine inlets signal air
masses originating from the Antarctic.
Grégory Fléchet, coordinator
Délégation à l’information et à la communication
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